1. Field of the Invention
The present invention relates to an optical head apparatus and an optical information processing apparatus for recording/reproducing or erasing information on an information storage medium such as an optical disk or an optical card.
2. Description of the Related Art
Optical memory technology that uses optical disks or optical cards as high-density, large-capacity information storage media has been expanding its application from digital audio disks to video disks, document file disks, and further to data files, etc. In this optical memory technology, information is recorded and reproduced with high accuracy and reliability by using a light beam focused into a microscopic spot.
The construction and operation of an optical head apparatus that uses a holographic device as a wavefront converting means, and that employs a spot size method as a method of focus error signal detection and a phase difference method as a method of tracking error signal detection, will be described as a prior art example of the optical head apparatus. FIG. 32 shows a side view of the prior art optical head apparatus. Light emitted from a semiconductor laser 101 as a light source is converted by a collimator lens 102 into a parallel beam of light, which is then reflected by a beam splitter 103 and converged by an objective lens 104 onto an information layer 131 on an optical disk 130 which is a single-layer information storage medium. The focal point of the light converged by the objective lens 104 is designated by F0. An actuator 107 moves the objective lens 104 together with its holding means 106 in such a manner as to follow disk movements associated with the surface warping and eccentricity of the optical disk 130.
The light diffracted and reflected by the information layer 131 is again passed through the objective lens 104 and becomes a parallel beam of light. This parallel beam of light passes through the beam splitter 103 and is converged by a detection lens 108. The converged light enters a holographic device 109 acting as awavefront converting means which, to generate a focus error signal, produces diffracted rays of light having their focuses at points respectively displaced along the direction of the optical axis. These diffracted rays of light are received by a photodetector 110. When the focal point F0 of the objective lens 104 is kept at the information layer 131 on the optical disk 130, the diffracted light that focuses at a point nearer to the detection lens 108 than a detection surface 110a of the photodetector 110 is equal, in terms of the cross sectional area on the detection surface 110a, to the diffracted light that focuses at a point on the side of the detection surface 110a opposite from the detection lens 108.
FIG. 33 shows a zone splitting pattern of the holographic device 109, detection regions 201 to 207 of the photodetector 110, and the cross sectional shapes of the diffracted rays on the detection surface 110a. The holographic device 109 is split into a large number of rectangular zones. Reflected beam 131a is diffracted by the holographic device 109. Here, -1st order diffracted light 131b is used for the detection of the focus error signal and +1st order diffracted light 131c for the detection of the tracking error signal. Symbols attached to the respective zones correspond to the symbols attached to the cross sections of the diffracted rays of light on the photodetector 110 in FIG. 33. The -1st order diffracted light 131b generated from the zones with the uppercase characters A to D is brought to a focus rearwardly of the detection surface 110a of the photodetector 110, as viewed from the detection lens 108. On the other hand, the -1st order diffracted light 131b generated from the zones with the lowercase characters a to d is brought to a focus forwardly of the detection surface 110a of the photodetector 110, as viewed from the detection lens 108. The holographic device 109 is so designed that when the focal point F0 of the objective lens 104 is at the information layer 131 on the optical disk 130, the detection spot designated by each uppercase character on the detection surface 110a of the photodetector 110 is equal in size to the detection spot designated by each lowercase character. Signals obtained according to the amounts of light received at the detection regions 201 to 203 are denoted by P1 to P3, respectively. The focus error signal FE is then obtained by calculating the following equation (equation 1). EQU FE=P1+P3-P2 (equation 1)
The +1st order diffracted light 131c is detected at the detection regions 204 to 207. Signals obtained according to the amounts of light received at the detection regions 204 to 207 of the photodetector 110 are denoted by t1 to t4, respectively. The tracking error signal TE according to the phase difference method is then obtained by comparing the phase of (t1+t3) with the phase of (t2+t4).
An RF signal for reproducing information is obtained by RFf or RFt in the following equation or by the sum of the two. EQU RFf=P1+P2+P3 EQU RFt=t1+t2+t3+t4
As the storage capacity of information storage media increases, there have been proposed multi-layer information storage media having on one side thereof multiple information layers capable of recording or reproducing information. One example of such media is the dual-layer DVD disk, in which two information layers are formed close to each other, and the reflectivity of the information layer nearer to the readout surface of the medium is reduced to transmit light while keeping the reflectivity of the information layer farther from the readout surface substantially at 100% as in the previous media, thereby making reproduction from one side possible.
The optical head apparatus that uses a holographic device to obtain focus and/or tracking error signals on a single-layer storage medium, as described above as the prior art example, has been implemented independently of multi-layer information storage media, and an optical head apparatus that uses a holographic device to obtain focus and/or tracking error signals on a multi-layer storage medium has not been implemented yet.
Further, the prior art optical head apparatus has the problem that when recording or reproducing information on a multi-layer storage medium capable of recording/reproduction from one side thereof, focus control becomes unstable by being affected by light reflected from an information layer other than the target information layer.
Next, a description will be given of a prior art circuit for obtaining the tracking error signal, which is different from the prior art example described above. FIG. 34 shows a circuit diagram of the prior art example that uses the phase difference method to obtain the tracking error signal. An adder 401 takes as inputs the signals t1 and t3 obtained according to the amounts of light received at the detection regions 204 and 206, and outputs a signal representing their sum. Similarly, an adder 402 accepts at its inputs the signals t2 and t4 obtained according to the amounts of light received at the detection regions 205 and 207, and outputs a signal representing their sum.
A phase difference signal generating circuit 308 accepts at its inputs the output signals from the adders 401 and 402, detects the temporal phase difference between these signals, and outputs a tracking error signal TE0. On the other hand, an adder 403 accepts at its inputs the output signals from the adders 401 and 402, and outputs a signal representing their sum. The output signal of the adder 403 is derived as the RF signal RFt for reproducing information.
A defect detection circuit 310 accepts at its input the output signal from the adder 403, and outputs a defect detection signal DED. The defect detection signal DED is output when there occurs a decrease in the input RF signal in a frequency band lower than the frequency band of the signal recorded on the optical disk 130 because of contamination, etc. of the surface of the optical disk 130.
A sample-and-hold circuit 311 takes the tracking error signal TE0 output from the phase difference signal generating circuit 308 and, when the defect detection signal DED indicates a defect detected state, outputs the tracking error signal TE0 held therein that was taken immediately before entering the defect detected state; on the other hand, when the signal DED indicates a no-defect detected state, the input signal is directly output.
The prior art circuit for obtaining the tracking error signal for the optical head apparatus by the phase difference method, however, has had the problem that the sensitivity of the tracking error signal due to the beam's deviation from a split line on the holographic device, when the beam is accurately focused on the track, rapidly degrades.
Also, the prior art optical head apparatus has had the problem that tracking control becomes unstable if the defect detection that is necessary when obtaining the tracking error signal is to be achieved by using the total amount of light output from the holographic device.
In addition to the above-described problems associated with the two prior art examples, there is also the problem that an offset is created in the focus error signal when the objective lens moves to follow the movement of the track that is caused by disk eccentricity, or when the spot on the disk moves across the track.
Next, a prior art optical information processing apparatus that uses the above-described optical head apparatus will be described for reference purposes. FIG. 35 shows the construction of an optical disk drive as the optical information processing apparatus. The optical disk 130 is rotated by a motor 501. The optical head apparatus 502 is moved between the inner and outer circumferential portions of the optical disk 130 by means of a head transport device 503. The optical head apparatus 502 shines a beam of light onto the optical disk 130, receives the reflected light, and outputs a signal proportional to it. A control circuit 504 receives the signal output from the optical head apparatus 502, and outputs a servo control signal to the optical head apparatus 502. A data generation circuit 505 receives the signal from the optical head apparatus 502, and reproduces the information recorded on the optical disk 130. If the optical disk drive has a recording facility, a signal to be recorded on the optical disk is generated from information input to the data conversion circuit and the signal is supplied to the optical head disk 502.